1. Field of the Invention
The present invention relates to an optical memory device which records, reproduces or erases information onto or from a recording medium by applying a light beam such as a laser beam to the recording medium. More particularly, the present invention is concerned with an optical head in such an optical memory device.
2. Description of the Related Art
Extensive studies have recently been made about the potential use of an optical memory device as a high density storage, a high capacity storage having high speed accessing. As examples of such an optical memory device there are a memory device in which a row of fine pits are formed in a recording medium and the diffraction phenomenon of light beam at the pit portions is utilized for the reproduction of information, and a memory device in which bit-shaped areas having different refractive indices are formed on a recording medium and variations in reflectance or transmissivity are utilized for the reproduction of information.
Those optical memory devices are each provided with an optical head, in which there are performed a focus control and a tracking control for a radiated light beam relative to a recording medium by means of a servo control unit. FIG. 1 shows a construction of an optical system for this optical head, in which an isolator is formed by a polarization beam splitter 104 and a quarter-wave plate 105 to prevent a light beam reflected from a recording medium 101 from returning to a laser device 102 and to conduct the reflected beam efficiently to a servo signal detector or information signal detector 103. In this construction, the portion of the reflected beam remaining on the surfaces of such optical elements as the beam splitter 104 and quarter-wave plate 105 (such residual reflected beam will hereinafter be referred to as "stray beam") mostly returns to the laser device 102, with only a very small quantity of light beam reaching the servo signal detector 103, which is negligible as disturbance influencing a servo signal in the case where the reflectance of a recording medium is high as in VD (Video Disc) or CD (Compact Disc).
However, where the reflectance of a recording medium is low, or in the case of an optical head in which it is difficult to introduce such an isolator as shown in FIG. 1, e.g. an optical head for an optomagnetic recording device, if the residual reflectance on the surface of an optical element cannot be suppressed below a certain level, a stray beam on the surface is added as offset to a servo signal, thus causing problems; for example, the control for the optical head becomes unstable or it becomes difficult to effect mounting and adjustment of a light detector for servo control.